State-of-the-Art Perovskite Solar Cells Benefit from Photon Recycling at Maximum Power Point

Photon recycling is required for a solar cell to achieve an open-circuit voltage ($V_{OC}$) and power conversion efficiency (PCE) approaching the Shockley-Queisser theoretical limit. In metal halide perovskite solar cells, the achievable performance gains from photon recycling remain uncertain due to high variability in perovskite material quality and the non-radiative recombination rate ($k_{1}$). In this work, we study state-of-the-art $\textrm{Cs}_{0.05}(\textrm{MA}_{0.17}\textrm{FA}_{0.83})_{0.95}\textrm{Pb}(\textrm{I}_{0.83}\textrm{Br}_{0.17})_{3}$ films and analyze the impact of varying non-radiative recombination rates on photon recycling and device performance. Importantly, we predict the impact of photon recycling at the maximum power point (MPP), demonstrating an absolute PCE increase of up to 2.0% in the radiative limit, primarily due to a 77 mV increase in $V_{MPP}$. Even with finite non-radiative recombination, benefits from photon recycling can be achieved when non-radiative lifetimes and external LED electroluminescence efficiencies measured at open-circuit, $Q_{e}^{LED}(\textrm{V}_{OC})$, exceed 2 $\mu$s and 10%, respectively. This analysis clarifies the opportunity to fully exploit photon recycling to push the real-world performance of perovskite solar cells toward theoretical limits.Comment: Main text: 16 pages and 6 figures, SI: 22 pages and 21 figure

Mapping the Diffusion Tensor in Microstructured Perovskites

Understanding energy transport in semiconductors is critical for design of electronic and optoelectronic devices. Semiconductor material properties such as charge carrier mobility or diffusion length are measured in bulk crystals and determined using models that describe transport behavior in homogeneous media, where structural boundary effects are minimal. However, most emerging semiconductors exhibit microscale heterogeneity. Therefore, experimental techniques with high spatial resolution paired with models that capture anisotropy and domain boundary behavior are needed. We develop a diffusion tensor-based framework to analyze experimental photoluminescence (PL) diffusion maps accounting for material microstructure. Specifically, we quantify both carrier transport and recombination in single crystal and polycrystalline lead halide perovskites by globally fitting diffusion maps, with spatial, temporal, and PL intensity data. We reveal a 29% difference in principal diffusion coefficients and alignment between electronically coupled grains for CH3NH3PbI3 polycrystalline films. This framework allows for understanding and optimizing anisotropic energy transport in heterogeneous materials.Comment: 47 pages, 19 figure

Charge-Carrier Recombination in Halide Perovskites.

The success of halide perovskites in a host of optoelectronic applications is often attributed to their long photoexcited carrier lifetimes, which has led to charge-carrier recombination processes being described as unique compared to other semiconductors. Here, we integrate recent literature findings to provide a critical assessment of the factors we believe are most likely controlling recombination in the most widely studied halide perovskite systems. We focus on four mechanisms that have been proposed to affect measured charge carrier recombination lifetimes, namely: (1) recombination via trap states, (2) polaron formation, (3) the indirect nature of the bandgap (e.g., Rashba effect), and (4) photon recycling. We scrutinize the evidence for each case and the implications of each process on carrier recombination dynamics. Although they have attracted considerable speculation, we conclude that multiple trapping or hopping in shallow trap states, and the possible indirect nature of the bandgap (e.g., Rashba effect), seem to be less likely given the combined evidence, at least in high-quality samples most relevant to solar cells and light-emitting diodes. On the other hand, photon recycling appears to play a clear role in increasing apparent lifetime for samples with high photoluminescence quantum yields. We conclude that polaron dynamics are intriguing and deserving of further study. We highlight potential interdependencies of these processes and suggest future experiments to better decouple their relative contributions. A more complete understanding of the recombination processes could allow us to rationally tailor the properties of these fascinating semiconductors and will aid the discovery of other materials exhibiting similarly exceptional optoelectronic properties.EPSRC DTP Studentshi

Slowed Recombination via Tunable Surface Energetics in Perovskite Solar Cells

Metal halide perovskite semiconductors have the potential to reach the optoelectronic quality of meticulously grown inorganic materials, but with a distinct advantage of being solution processable. Currently, perovskite performance is limited by charge carrier recombination loss at surfaces and interfaces. Indeed, the highest quality perovskite films are achieved with molecular surface passivation, for example with n-trioctylphosphine oxide, but these treatments are often labile and electrically insulating. As an alternative, the formation of a thin 2D perovskite layer on the bulk 3D perovskite reduces non-radiative energy loss while also improving device performance. But, thus far, it has been unclear how best to design and optimize 2D/3D heterostructures and whether critical material properties, such as charge carrier lifetime, can reach values as high as ligand-based approaches. Here, we study perovskite devices that have exhibited power conversion efficiencies exceeding 25% and show that 2D layers are capable of pushing beyond molecular passivation strategies with even greater tunability. We set new benchmarks for photoluminescence lifetime, reaching values > 30 {\mu}s, and perovskite/charge transport layer surface recombination velocity with values < 7 cm s^{-1}. We use X-ray spectroscopy to directly visualize how treatment with hexylammonium bromide not only selectively targets defects at surfaces and grain boundaries, but also forms a bandgap grading extending > 100 nm into the bulk layer. We expect these results to be a starting point for more sophisticated engineering of 2D/3D heterostructures with surface fields that exclusively repel charge carriers from defective regions while also enabling efficient charge transfer. It is likely that the precise manipulation of energy bands will enable perovskite-based optoelectronics to operate at their theoretical performance limits.Comment: Main text: 15 pages, 4 figures. Supporting Information: 31 pages, 19 figure

Photo-induced halide redistribution in organic-inorganic perovskite films.

Organic-inorganic perovskites such as CH3NH3PbI3 are promising materials for a variety of optoelectronic applications, with certified power conversion efficiencies in solar cells already exceeding 21%. Nevertheless, state-of-the-art films still contain performance-limiting non-radiative recombination sites and exhibit a range of complex dynamic phenomena under illumination that remain poorly understood. Here we use a unique combination of confocal photoluminescence (PL) microscopy and chemical imaging to correlate the local changes in photophysics with composition in CH3NH3PbI3 films under illumination. We demonstrate that the photo-induced 'brightening' of the perovskite PL can be attributed to an order-of-magnitude reduction in trap state density. By imaging the same regions with time-of-flight secondary-ion-mass spectrometry, we correlate this photobrightening with a net migration of iodine. Our work provides visual evidence for photo-induced halide migration in triiodide perovskites and reveals the complex interplay between charge carrier populations, electronic traps and mobile halides that collectively impact optoelectronic performance

The Impact of Atmosphere on the Local Luminescence Properties of Metal Halide Perovskite Grains.

Metal halide perovskites are exceptional candidates for inexpensive yet high-performing optoelectronic devices. Nevertheless, polycrystalline perovskite films are still limited by nonradiative losses due to charge carrier trap states that can be affected by illumination. Here, in situ microphotoluminescence measurements are used to elucidate the impact of light-soaking individual methylammonium lead iodide grains in high-quality polycrystalline films while immersing them with different atmospheric environments. It is shown that emission from each grain depends sensitively on both the environment and the nature of the specific grain, i.e., whether it shows good (bright grain) or poor (dark grain) luminescence properties. It is found that the dark grains show substantial rises in emission, while the bright grain emission is steady when illuminated in the presence of oxygen and/or water molecules. The results are explained using density functional theory calculations, which reveal strong adsorption energies of the molecules to the perovskite surfaces. It is also found that oxygen molecules bind particularly strongly to surface iodide vacancies which, in the presence of photoexcited electrons, lead to efficient passivation of the carrier trap states that arise from these vacancies. The work reveals a unique insight into the nature of nonradiative decay and the impact of atmospheric passivation on the microscale properties of perovskite films

Efficient perovskite solar cells by metal ion doping

Realizing the theoretical limiting power conversion efficiency (PCE) in perovskite solar cells requires a better understanding and control over the fundamental loss processes occurring in the bulk of the perovskite layer and at the internal semiconductor interfaces in devices. One of the main challenges is to eliminate the presence of charge recombination centres throughout the film which have been observed to be most densely located at regions near the grain boundaries. Here, we introduce aluminium acetylacetonate to the perovskite precursor solution, which improves the crystal quality by reducing the microstrain in the polycrystalline film. At the same time, we achieve a reduction in the non-radiative recombination rate, a remarkable improvement in the photoluminescence quantum efficiency (PLQE) and a reduction in the electronic disorder deduced from an Urbach energy of only 12.6 meV in complete devices. As a result, we demonstrate a PCE of 19.1% with negligible hysteresis in planar heterojunction solar cells comprising all organic p and n-type charge collection layers. Our work shows that an additional level of control of perovskite thin film quality is possible via impurity cation doping, and further demonstrates the continuing importance of improving the electronic quality of the perovskite absorber and the nature of the heterojunctions to further improve the solar cell performance

Charge-carrier dynamics and mobilities in formamidinium lead mixed-halide perovskites

The mixed‐halide perovskite FAPb(BryI1–y)3 is attractive for color‐tunable and tandem solar cells. Bimolecular and Auger charge‐carrier recombination rate constants strongly correlate with the Br content, y, suggesting a link with electronic structure. FAPbBr3 and FAPbI3 exhibit charge‐carrier mobilities of 14 and 27 cm2 V−1 s−1 and diffusion lengths exceeding 1 μm, while mobilities across the mixed Br/I system depend on crystalline phase disorder